The inventive method and the device for carrying out the method are applicable for adhesive binding, i.e. for joining with the help of an adhesive a plurality of stacked product parts on one stack surface for producing a finished product with a glued spine, whereby one product part may be one single sheet, a sheet folded once or several times or several sheets folded once and positioned inside each other. The products consist of fibrous material, e.g. paper. The products produced by means of adhesive binding are e.g. newspapers, brochures, books or similar products. The glued spine may additionally be covered with a spine strip which may be part of the finished product.
As long as single sheets or product parts which are once folded, i.e. two pages connected by a folded edge, are to be joined by a glued spine this can be carried out without great expense. It is sufficient to introduce the adhesive between the product parts such that these on the one hand are held together sufficiently, and on the other hand such that opening of the finished product is not impaired too much, whereby a minimal impediment cannot be avoided.
If however, multi-layer product parts which are folded several times, e.g. several times folded sheets or a plurality of sheets positioned inside each other, are to be joined by means of a glued spine this causes a further problem which must be solved: in addition to the fact that the edges or folded edges of the product parts must be joined, the adhesive action must also reach the inner edges of the folds through the outer edges such that the inner parts of each product part are also joined to the glued spine.
According to the state of the art glued spines on products consisting of multi-layer product parts are produced by milling off the spine area formed by the folded edges of the multi-layer product parts to such a depth that all folded edges (possibly not including the innermost folded edge) are milled off and thus actually a stack of single sheets is formed which is then glued in the same way as any stack of single sheets. Between milling and gluing, usually dust is removed from the spine area. These methods are commonly used although the required devices are rather costly and they comprise at least two disadvantages: firstly large amounts of waste in form of shavings is produced and must be sucked away, secondly by milling off the folded edges, the individual pages of each product part lose their inherent connection given by the folded edge and therefore the adhesive connection must be considerably stronger than a similar connection for pages which are at least partly connected by folded edges.
The two method disadvantages mentioned above are compensated by other known methods in which not the complete spine area is milled but only parts of it which parts expand over the complete width of the spine area (in the direction of the thickness of the finished product) and are normally spread regularly over the height of the spine (length of the edges which are to be joined by the glued spine). If such part areas are glued they have the same effect as staples or stitching thread with which sheets which are folded inside each other are also connected in specific points only. Between the glued portions, the folded edges are unchanged and thus the connection between two individual pages is maintained in the finished product and contributes to the stability of the end product.
Methods according to which multi-layer folded edges are drilled or punched and then glue is inserted into the openings lead to similar products. Such produced contact points (adhesive stitching) have the same effect as stitching and hold the individual layers of the product parts together such that these adhesively stitched product parts can then be joined with an adhesive spine in a following method step or integrated in the adhesive stitching step. Adhesive stitching methods are e.g. described in publications EP-0628429, EP-0664226 and EP-0662440. According to publication EP-390734 the adhesive is shot through the layers without the necessity of producing an opening with a corresponding tool.
Furthermore, the publications EP-390733 (or U.S. Pat. No. 5,350,268) and EP-409770 (or U.S. Pat. No. 5,193,851) describe a method in which a plurality of once folded sheets is collected, whereby a momentary outermost folded edge is treated with glue between the individual collecting steps such that the folded edge positioned on it in the next collecting step adheres to it thus forming product parts similar to stitched product parts which are then joined by a glued spine in the same way as once folded single sheets.